In image-guided operation systems, objects—for example, medical instruments or bones—are provided with marker devices in order to be able to ascertain the location, i.e. the position and/or alignment, of the object. The marker devices are usually one or more diffusely reflective spheres which reflect the light emitted from a light source. The reflected light is detected by a sensor, for example a 3D camera. The centre point of each reflection is ascertained in a computational unit and regarded as the centre point of a sphere for the subsequent calculation of the location. One problem occurs when a sphere is partially covered and the sensor can therefore only partially detect the reflection. The centre point of the reflection is also assumed as the centre point of the sphere in this case, but does not correspond to the actual centre point of the sphere. This usually leads to an error in calculating the location of the object.
This is avoided by using a retro-reflector in the marker device. A retro-reflector reflects back incident radiation, for example light, parallel to its direction of incidence. This occurs due to multiple reflection in the retro-reflector, wherein each incident beam experiences a parallel shift. Because of this parallel shift, the radiation path as a whole is interrupted as soon as the retro-reflector is covered over too great an area. Accordingly, the retro-reflector can only be detected by the sensor when it is sufficiently visible and can therefore be correctly localized.
It is often necessary in an image-guided operation system for the retro-reflector to possess retro-reflective properties all around, i.e. in all spatial directions. Such a retro-reflector is for example disclosed in the published patent application specification US 2008/0131115 A1. The retro-reflector shown in this document consists of eight cube corners, wherein the tips of the cube corners are adjacent to each other and each cube corner is formed from three reflective faces. The retro-reflector serves to detect a sound field, by modulating the retro-reflective radiation using a membrane situated in one of the reflective faces.